Microlithographic manufacturing systems and microscopic inspection systems continue to evolve for imaging smaller and smaller feature sizes. Ultimately, the minimum feature size depends upon the wavelength of the illumination. Shorter wavelengths are required for smaller feature sizes. Imaging systems now successfully operate at wavelengths as small as 193 nanometers, but the next generation of microlithographic imaging is expected to operate at even smaller wavelengths of around 157 nanometers. Microscopic inspection systems are also being developed to operate at the smaller wavelengths.
Not many materials have appropriate optical characteristics for producing images at wavelengths of light within the deep-ultraviolet spectrum near 157 nanometers in wavelength. The most promising candidate is calcium fluoride (CaF2), which has a cubic crystalline structure. However, calcium fluoride (CaF2) has been found to exhibit an intrinsic birefringence at 157 nanometer wavelengths. No birefringence is evident for light rays normal to the crystal's {111} planes; but rays with angular departures, which involve transmissions through other planes (e.g., {110}, {101}, and {011} planes), produce birefringence that increases at different rates in different directions. Peak birefringence is apparent in three evenly spaced directions, which is referred to as “three-fold symmetry”.
A number of solutions have been proposed to avoid or reduce the intrinsic birefringence. Some work is underway mixing crystalline materials with opposite birefringence, such as mixing barium fluoride (BaF2) with calcium fluoride, to produce a compound crystalline structure that is free of birefringence through a range of directions. Another approach assembles optical elements made of different materials having opposite signs of intrinsic birefringence for diminishing cumulative effects of birefringence.
A single birefringent material (e.g., calcium fluoride) has also been proposed for use among multiple optics that are relatively varied in angular orientation around an optical axis to balance the effects of birefringence in different directions. This procedure is referred to as clocking. For example, the peak birefringence of calcium fluoride occurs in three evenly spaced directions around the axis of the (111) plane. Successive optics can be arranged with their (111) plane's axis aligned with a common optical axis but each angularly rotated by different amounts around the common optical axis (i.e., clocked) to evenly distribute the directionally sensitive birefringence. Similar clocking effects can be achieved by orienting other of the crystal's planes, such as the crystal's {001} or {110} planes, normal to the optical axis and rotating successive optics by different amounts around the optical axis.
Imaging systems containing optics made from amorphous materials can also exhibit similar birefringence effects, particularly where light rays depart significantly from the optical axis, such as in systems with high numerical aperture. Such birefringence is generally evenly distributed around the optical axis.